From the scene

"Police are refusing to identify if anyone is a suspect yet. They are merely saying that they are speaking to a large number of people - family members, neighbours, people who have known the family for a long period of time.

"They say it's going to be a painstaking and lengthy investigation, involving police that have been flown up from Brisbane and Townsville, officers with specialist skills and officers who specialise in homicide investigations.

People here are absolutely devastated. We saw many people turn up to the crime scene, people who were family of the dead children, so distraught, such confronting scenes of people in terrible grief.

"There have also been a large number of people who I guess didn't know the family who have still turned out with floral tributes. Also people who have been waiting around the area of the crime scene, watching to see what's happening.

"People I've spoken to have said that it's a terrible and shocking crime, and particularly sad so close to Christmas."

- ABC reporter Sharnie Kim

Local MP Gavin King said he did not know the family directly but lived nearby.

"It's a street that's only a couple of minutes' drive from my own family home, and having been there so many times and attending community concerts in a park across from where this tragedy occurred ...

"It's just a tragedy that is really very difficult to come to terms with."

Cairns Mayor Bob Manning offered his condolences to the family and friends of the victims and asked the community to "unite and display sympathy for those directly affected by this incident and respect for the police and other authorities who now must go about this difficult work".

Premier Campbell Newman said he was deeply saddened and shocked by the tragedy.

"My thoughts and prayers are with the family and friends of those concerned," he said in a statement.

"I'm also aware of the impact this will have on the emergency service workers and police officers who responded to the scene, and I have asked the Acting Ministers to make sure those individuals are receiving the support they need.

"Indeed, the whole Cairns community and the people of Queensland will feel the effects of this tragedy, particularly at a time of year when families come together."

Prime Minister Tony Abbott said news of the "unspeakable crime" was "heartbreaking".

"All parents would feel a gut-wrenching sadness at what has happened," he said in a statement.

"These are trying days for our country. There will be tears and prayers across our country for these children."